Sorting based on a calculated set of values

I'm trying to sort output based on a value calculated in the XSL.

If my XML looks like the following:

<item id="foo">
<a>1</a>
<b>4</b>
</item>
<item id="goo">
<a>3</a>
<b>3</b>
</item>

And my out put is an HTML table table with four columns:
Col 1: the item id
Col 2: the a value
Col 3: the b value
Col 4: the product of a and b

Can I sort my output by the fourth column?  I know how to sort by the item
id, a, or b, but don't know what I need to do to properly address the
calculated value "column".

Thanks in advance,
Andrew



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